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Fighting poachers Wildlife crime investigators hope to crack down on illegal elephant killing with a new tool that uses nuclear test residue to determine the age of a tusk.

Tens of thousands of elephants are hunted for their ivory each year. As few as 470,000 African elephants remain, making them a vulnerable species while the Asian elephant is endangered and may number about 30,000, experts say.

Despite international agreements that ban most raw ivory trade from Asian elephants after 1975, and African elephants after 1989, the slaughter continues in large part because police lack the means to tell the age of the ivory.

"We've developed a tool that allows us to determine the age of a tusk or piece of ivory, and this tells us whether it was acquired legally," says Kevin Uno, lead author of the study in the Proceedings of the National Academy of Sciences.

"Our dating method is affordable for government and law enforcement agencies and can help tackle the poaching and illegal trade crises," says Uno, a postdoctoral researcher at Columbia University.

The test costs about $500 a sample and uses a technique of analysing the amount of carbon-14 in the animal tissues.

Carbon-14 was formed in the atmosphere by above-ground nuclear tests in the 1950s and 1960s, conducted by the United States in Nevada and the Soviet Union in Siberia.

Levels peaked in the 1960s and have been declining ever since. The test devised by scientists should be effective for about another 15 years, by which point the atmospheric levels of carbon-14 will return to pre-nuclear-test norms.

Carbon dating

Researchers tested their technique on 29 animal and plant tissues -- including elephant tusks, hippo tusks, canine teeth and monkey hair as well as grass from Kenya -- each collected on known dates from 1905 to 2008.

They found that various tissues that formed at the same time had the same levels if carbon-14.

The four oldest samples were from animals that died from 1905 to 1953, and they had the least carbon-14 because they died before atmospheric nuclear weapons tests.

"With an accurate age of the ivory, we can verify if the trade is legal or not," says Uno.

"Currently 30,000 elephants a year are slaughtered for their tusks, so there is a desperate need to enforce the international trade ban and reduce demand."

African elephants are in "sharp decline" due to illegal poaching for the international ivory trade and habitat loss. Asian elephants are endangered with a population ranging from 25,600 to 32,750, according to the World Wildlife Fund.

About 70 percent of smuggled ivory is sold in China, and the United States is the second largest market.